examples.tests.simplewarning: Use -e to exit on failure

Use -e to exit on first failure.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

examples/tests/simplewarning.sh

-#!/bin/sh
+#!/bin/sh -e
 PATH=$(avocado "exec-path"):$PATH
 
 avocado_debug "Debug message"